Method of fabricating semiconductor device

ABSTRACT

A semiconductor device having a bottom electrode, a ferroelectric film, and a top electrode formed on a semiconductor substrate, wherein the angle of each of the main cross sectional sides of the ferroelectric film relative to the main surface of the semiconductor substrate is less than 75 degrees. Forming the ferroelectric film into the trapezoid in cross section having such an angle provides a microscopic capacitor without electrical short-circuit between the top and bottom electrodes if the top electrode, the ferroelectric film, and the bottom electrode are etched with single photolithography process step. The novel technique implements a microscopic memory cell structure suitable for highly integrated memory devices.

[0001] This application is a Continuation application of Ser. No.09/475,033, filed Dec. 30, 1999, which is a Divisional application ofSer. No. 08/755,602, filed Nov. 25, 1996, the contents of which areincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a semiconductor device havingcapacitors using a ferroelectric film such as ferroelectric nonvolatilememory of a dynamic random access memory (DRAM).

[0003] Some ferroelectric materials have extremely large relativedielectric constants ranging from several hundreds to several thousands.Therefore, use of a thin film made of these ferroelectric materials fora capacitor dielectrics provides a capacitor of small area and largecapacity suitable for large scale integration (LSI) devices. Also, theferroelectric material has spontaneous polarization that can be invertedin direction by an applied electric field, thereby providing anonvolatile memory.

[0004] As described in Japanese unexamined Patent Application No.5-90606 and referring to FIG. 14, the conventional ferroelectric memoryis fabricated by forming on an interlayer insulating film 144 with abottom Pt electrode 145, ferroelectric film 146, and a top Pt electrode147 in this order, thereby forming a ferroelectric capacitor. However,in the conventional ferroelectric memory, each of the layers is formedwith an independent mask, which makes the memory cell area large becauseof critical dimension uniformity and alignment tolerance, thereby makingit difficult to fabricate highly integrated memory devices. Theconventional technique also involves a problem of thinning theinterlayer insulating film 144 for the conventional technique repeatsthe patterning on it for forming the ferroelectric capacitors.

[0005] To solve the above-mentioned problems, a method was proposed asdescribed in Japanese unexamined Patent Application No. 2-288368, inwhich a top electrode 158, a ferroelectric film 157, and a bottomelectrode 156 are collectively dry-etched with the photoresist used as amask as shown in FIG. 15. This method uses polysilicon for the top andbottom electrodes 158 and 156, which are dry-etched with C₂Cl₂F₄, SF₆,and Ar gases.

[0006] However, forming a ferroelectric film directly on polysilicon, asilicon oxide film of a low dielectric constant is formed at theinterface. The silicon oxide film thus formed significantly deterioratescapacitor characteristics. To avoid this deterioration, it is necessaryto use electrodes made of noble metals such as platinum and palladium orconductive oxides such as IrO₂, RuO₂, and ReO₃.

[0007] Of the above-mentioned electrode materials, platinum isconsidered best suited for the application. Therefore, in the memorycell forming process described in Japanese unexamined Patent ApplicationNo. 5-299601 collectively dry-etches a top electrode 45, a ferroelectricfilm 44, a bottom electrode 43, and a conductive diffusion barrier layer169 with the photoresist used as the mask as shown in FIG. 16. Use ofsuch a structure can implement microscopic capacitors without losingtheir properties

[0008] Actually, however, platinum cannot be converted to a highlyvolatile reaction product to be dry-etched. It was observed that, ifplatinum is dry-etched, a redeposited material forms a wall-shapedresidue (hereinafter referred to as a platinum-contained deposit) on thecapacitor side wall due to the low volatility. In this structure, theabove-mentioned platinum-contained deposits short-circuit the topelectrode 45 and the bottom electrode 43.

[0009] It is therefore an object of the present invention to provide acapacitor in which the top and bottom electrodes thereof will not beshort-circuited when the top electrode, the ferroelectric film, and thebottom electrode are etched with single photolithography process step.

SUMMARY OF THE INVENTION

[0010] This object is achieved by setting the taper angle of the sidewall of the ferroelectric film constituting the ferroelectric capacitorto less than 75 degrees to the main surface of the substrate on whichthe ferroelectric capacitor is formed. That is, the taper angle of thecross sidewall of the ferroelectric capacitor to the plane on which thebottom electrode is formed is set to a value not reaching 75 degrees ormore.

[0011] Referring to FIG. 13, there is shown a relationship between thetaper angle of the cross side wall of the ferroelectric capacitor to themain surface of the substrate and short-circuit. It is assumed hereinthat a short-circuit has occurred when a leakage current density at anapplied voltage of 3V became 10⁻⁵ A/cm² or higher. In theabove-mentioned prior art, the etching is performed at nearly 90degrees, so that, after etching of the platinum top electrode 45, theplatinum of the top electrode 45 redeposits to form a platinum-containedsidewall deposit 101 as shown in FIG. 10A. After completion ofdry-etching of the ferroelectric film 44, a sidewall deposit 102composed of elements constituting the ferroelectric film 44 remainsalong the platinum-contained sidewall deposit 101 as shown in FIG. 10B.Although this sidewall deposit 102 is composed of the components of theferroelectric film, the composition and crystal structure thereof areout of order, resulting in insufficient insulation. Referring to FIG.10C, during etching of the platinum bottom electrode 43, this deposit102 composed of the components of the ferroelectric film is mostlyremoved. However, the platinum-contained sidewall deposit 101 stillremains. Further, the platinum-contained sidewall deposit 103 may alsobe formed from the platinum bottom electrode. Thus, in the prior-arttechnology, depositing of platinum on the sidewall short-circuits thebottom and top electrodes 43 and 45 of the capacitor.

[0012] Referring to FIG. 13, it is clear that setting the angle of thecross sidewall of the platinum bottom electrode, the ferroelectric filmand the top electrode to the main surface of the substrate to less than75 degrees prevents the platinum deposits from being formed on thecapacitor sidewall.

[0013] In FIG. 13, the angle of the cross sidewall of the platinumbottom electrode, the ferroelectric film, and the top electrode to themain surface of the substrate is shown; however, it is not alwaysnecessary to set the cross sectional sidewall of the entire capacitor toless than 75 degrees. For example, tilting the sidewall of only theferroelectric film 44 relative to the main surface of the substrate byless than 75 degrees also provides an effect of preventing the platinumdeposition from occurring. The effect can be made more conspicuous,however, by tilting together the sidewall of the platinum bottomelectrode by less than 75 degrees.

[0014] It will be apparent that, instead of platinum, the top electrode45 may be another rare metal such as iridium or ruthenium or aconductive oxide such as IrO₂, RuO₂, or ReO₃. If platinum is not usedfor the top electrode 45, the platinum-contained deposit is formed onthe capacitor sidewall only when the platinum bottom electrode 43 isetched. As described above, tapering the capacitor side walls to themain surface of the substrate by less than 75 degrees prevents theshort-circuit between the top electrode and the platinum bottomelectrode.

[0015] The angle of the cross sidewall of the ferroelectric capacitor tothe bottom surface of the bottom electrode is determined by the angle ofthe etching mask sidewall to the bottom surface of the bottom electrode.In the present invention, tungsten is used for the etching mask. Whentungsten is etched by anisotropic dry etching, the angle of the tungstensidewall to the bottom surface of the bottom electrode is determined bythe angle of the photoresist side walls. FIG. 11 shows a relationshipbetween the sidewall taper angle of photoresist sidewall and resistbaking temperature. Shown are test results obtained from two types ofphotoresists A and B. The results indicate that the sidewall taper anglegets larger as the baking temperature rises for both the photoresists.The photoresist A is composed of a material having a flat distributionover molecular weights of 100 to 30,000, while the photoresist B iscomposed of a material having a peak over molecular weights 2,000 to3,000. For the photoresists shown, a preferable result is obtained bysetting the baking temperature to a range of 140° C. to 160° C. Themethod of controlling the sidewall taper angle by the resist bakingtemperature is also applicable to the case in which materials such asSiO₂ for which isotropic tapering is difficult is used for the etchingmask.

[0016] When tungsten is etched by isotropic dry etching, the angle ofthe tungsten sidewall to the bottom surface of the bottom electrode canbe controlled by the over-etching time of tungsten. FIG. 12 shows arelationship between the over-etching time of tungsten and the angle ofthe tungsten sidewall to the main surface of the substrate. As thetungsten over-etching time is increased, line width becomes narrower,while the sidewall approaches vertical angle. A preferable result willbe obtained when the tungsten over-etching time is set to a range of 5%to 10%.

[0017] However, when etching the ferroelectric capacitor such that thesidewall taper angle thereof becomes less than 75 degrees relative tothe main surface of the substrate, the ferroelectric sidewall is exposedto plasma, which may cause an etching damage, resulting in an increasein the leakage current on the sidewall. This problem is overcome byperforming oxygen plasma processing after dry-etching of the bottomelectrode and before etching the conductive diffusion barrier layer(hereinafter referred to simply as the diffusion barrier layer).

[0018] It should be noted that performing oxidization processing foretching damage recovery after etching TiN of the diffusion barrier layeroxidizes the TiN under the bottom platinum electrode to cause peel-offor the like trouble. The peel-off can be prevented from occurring byperforming oxygen plasma processing before etching the TiN.

[0019] These above and further objects and features of the inventionwill be seen by reference to the description, taken in connection withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In the drawings, forming a part thereof, in which like referencecharacters denote like parts in the various views.

[0021]FIG. 1 is a cross section illustrating a semiconductor deviceaccording to the present invention;

[0022]FIG. 2 is a cross section illustrating a process of fabricating amemory cell based on the present invention;

[0023]FIG. 3 is a cross section illustrating a fabrication process nextto that of FIG. 2;

[0024]FIG. 4 is a cross section illustrating a fabrication process nextto that of FIG. 3;

[0025]FIG. 5 is a cross section illustrating a fabrication process nextto that of FIG. 4;

[0026]FIG. 6 is a cross section illustrating a fabrication process nextto that of FIG. 5;

[0027]FIG. 7 is a cross section illustrating a fabrication process nextto that of FIG. 6;

[0028]FIG. 8 is a cross section illustrating a fabrication process nextto that of FIG. 7;

[0029]FIG. 9 shows characteristics curves indicating a relationshipbetween the voltage and leakage current density of the capacitoraccording to the invention after the fabrication processes of FIGS. 6through 8;

[0030]FIG. 10A is a cross section illustrating sidewall deposits formedon the capacitor as observed after dry-etching the top electrode;

[0031]FIG. 10B is a cross section illustrating sidewall deposits formedon the capacitor as observed after dry-etching the ferroelectric film;

[0032]FIG. 10C is a cross section illustrating sidewall deposits formedon the capacitor as observed after dry-etching the bottom electrode;

[0033]FIG. 11 shows characteristics curves indicating a relationshipbetween baking temperature and photoresist sidewall taper angle;

[0034]FIG. 12 shows characteristics curves indicating a relationshipbetween over-etching time and tungsten mask taper angle;

[0035]FIG. 13 shows capacitor short-circuit test results;

[0036]FIG. 14 is a cross section illustrating a prior-art memory cell;

[0037]FIG. 15 is a cross section illustrating another prior-art memorycell; and

[0038]FIG. 16 is a cross section illustrating still another prior-artmemory cell.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

[0039] Now, referring to FIGS. 2 through 8, there are shown crosssections illustrating a method of fabricating a memory of FIG. 1 in theorder of main processes, the method being practiced as one preferredembodiment of the present invention.

[0040] First, as shown in FIG. 2, a switching transistor is formed bythe conventional MOSFET (Metal Oxide Semiconductor Field-EffectTransistor) forming process. In the figure, reference numeral 21indicates a p-type semiconductor substrate, reference numeral 22indicates an isolation dielectric, reference numeral 23 indicates a gateoxide film, reference numeral 24 indicates a word line that provides agate electrode, reference numerals 25 and 26 indicate phosphorous-dopedn-type regions, and reference numeral 27 indicates an interlayerinsulating film. A deposit of SiO₂ 28 of about 600 nm in thickness isformed by the known CVD (Chemical Vapor Deposition) all over thetransistor. Then, the formed deposit is reflowed at 850° C. to be etchedback by about 300 nm, thereby smoothing steps caused by the word line.

[0041] Next, an opening is formed in the SiO₂ 28 so that a bit line cancome into contact with the n-type region 25. The opening is made byknown photolithography and dry etching techniques. Then, the bit line 31is formed. This bit line 31 is composed of a film stack made of metalsuicide and polysilicon. This film stack is etched by the knownphotolithography and dry etching to a desired bit-line pattern.

[0042] An insulating film 32 of silicon oxide film type such as BPSG(Boron-doped Phosphor-Silicate Glass) is deposited to planarize. Itshould be noted that this insulating film 32 needs to be thick enoughfor to planarize the substrate surface. In the present embodiment, theinsulating film 32 was formed to a thickness of about 600 nm andplanarized by etching back.

[0043] Referring to FIG. 3, a contact hole 33 of memory section isopened to provide access for the storage capacity section to come intocontact with the substrate. On the insulating film 32 and inside thecontact hole 33, polysilicon 41 was deposited by CVD to a thickness ofabout 350 nm. Then, by dry etching, the polysilicon 41 was etched backby the film thickness to fill the contact hole 33 as shown in FIG. 4.

[0044] By sputtering, TiN is formed to a thickness of about 50 nm as adiffusion barrier layer 42 and then a bottom electrode 43 is formed. Inthe present embodiment, a Pt film about 200 nm thick was deposited asthe bottom electrode 43. The TIN of the diffusion barrier layer 42 isprovided to prevent the platinum of the bottom electrode 43 and theoxygen from diffusing into the polysilicon 41. Then, a ferroelectricfilm 44 is formed. In the present embodiment, a lead zirconate titanate(Pb(Zr_(0.5)Ti_(0.5))O₃) film was formed to a thickness of about 150 nmby reactive evaporation and then crystallized by heat treatment inoxygen atmosphere at 650° C. for 30 seconds for obtaining theferroelectric film 44. It will be apparent that the ferroelectric film44 may also be formed by high-frequency magnetron sputtering, Sol-Gelmethod, MOD (Metal Organic Decomposition), or CVD. Then, by sputtering,a Pt film about 50 nm thick was formed as a top electrode 45 and atungsten 46 was deposited to a thickness of about 350 nm for the mask asshown in FIG. 4.

[0045] Referring to FIG. 5, the tungsten 46 is patterned by dry-etchingwith SF₆, a photoresist 51 being used as the mask. After the photoresist51 has been removed, the top electrode 45 is patterned by the sputteretching with the tungsten 46 used as the mask as shown in FIG. 6. Indoing so, the dry etching conditions were adjusted such that isotropicetching is provided, and the etching was performed such that the crosssection of the tungsten 46 becomes a trapezoid, the angle of each of thesides thereof relative to the substrate being less than 75 degrees. Inthe present embodiment, microwave dry etching was used with theconditions that an SF₆ gas flow of 10 SCCM, a pressure of 2 mTorr, and amicrowave power of 400 W.

[0046] If the dry etching is performed with high anisotropy, the crosssection of the tungsten 46 departs from the trapezoid and approaches arectangular, leaving projecting deposits on the tungsten 46 and thephotoresist 51 at the sidewalls thereof. As shown in FIG. 10A, after thephotoresist 51 has been removed, platinum-contained projecting sidewalldeposits 101 remain. As shown in FIG. 10B, after the ferroelectric film44 has been dry-etched, sidewall deposits 102 composed of the componentsof the ferroelectric film remain on the periphery of theplatinum-contained projecting sidewall deposits 101.

[0047] When the ferroelectric film 44 has been etched by use of a mixedgas composed of CF₄ and Ar, the bottom electrode 43 is etched by sputteretching. It should be noted that 30% over-etching was performed in orderto remove the platinum deposited on the sidewalls as shown in FIG. 7.

[0048] Oxygen plasma is generated in the same chamber in which theabove-mentioned etching was performed to recover the etching damages ofthe cross section of the ferroelectric film 44. In the presentembodiment, the oxygen plasma processing was performed at an oxygen flowof 25 SCCM, a pressure of 30 mTorr, an RF power of 150 W for threeminutes as shown in FIG. 8. Then, by the dry etching with SF₆, thediffusion barrier layer 42 and the remaining tungsten 46 were removedsimultaneously to complete the ferroelectric capacitor of FIG. 1.Although not shown, the memory device is completed by performing wiringlike an ordinary semiconductor memory chip.

[0049] According to the above-mentioned processes, the increase inleakage current on the sidewalls and the decrease in breakdown voltagecan be prevented at the same time, thereby allowing fabrication ofmicroscopic ferroelectric memory cells suitable for high integration.The leakage of the ferroelectric capacitor can also be decreased byperforming the oxygen plasma processing after etching the diffusionbarrier layer 42 by the dry etching using SF₆. However, this causesoxidization of the diffusion barrier layer 42 left under the bottomelectrode 43 from the sides of the layer, resulting in peeling off ofthe bottom electrode 43 from the bottom electrode/diffusion barrierlayer interface. This problem can be avoided if the oxygen plasmaprocessing is performed before dry-etching the diffusion barrier layer42.

[0050]FIG. 9 shows comparisons of the leakage current density andvoltage characteristics of the ferroelectric capacitor obtained afterthe etching of the top electrode shown in FIG. 6, after the etching ofthe bottom electrode shown in FIG. 7, after the over etching, after theoxygen plasma processing of FIG. 8, and after the etching of thediffusion barrier layer. After the top electrode etching, the leakagecurrent density is on a order of 10⁻⁷ A/cm². After the bottom electrodeetching, the platinum-contained deposits on the ferroelectric filmsidewalls short-circuit the top and bottom electrodes. Removing theseplatinum-contained deposits by 30% over-etching decreases the leakagecurrent density to an order of 10⁻⁵ A/cm². However, this value is largerthan that obtained after the top electrode etching by an order ofmagnitude or more because the ferroelectric film sidewalls are exposedto the plasma to cause oxygen defects. When the oxygen defects on theferroelectric film sidewalls are remedied by oxygen plasma processing,the leakage current density decreases to the generally same level asthat observed after the top electrode etching. Etching of the diffusionbarrier layer does not indicate an increase in the leakage currentdensity either.

[0051] In the present embodiment, the tungsten 46 that provides the maskis formed by isotropy dry etching into a trapezoid in cross section. Itwill be apparent that, as described above with reference to FIG. 11, thetungsten 46 maybe formed by anisotropic dry etching into a trapezoid incross section after forming the photoresist 51 into a trapezoid in crosssection by setting the baking temperature of the photoresist 51 to arange of 140° C. to 160° C.

[0052] In the present embodiment, lead ziroconate tintanate is used forferroelectric film 44. It will be apparent that the material for theferroelectric film is not limited to lead zirconate titanate; alsoavailable are, by way of example, perovskite-type oxides such as leadtitanate, strontium titanate, and barium titanate, solid solutions ofthese, and bismuth-type layer-structured ferroelectric oxides.

[0053] In the present embodiment, TiN is used for the diffusion barrierlayer 42. It will be apparent that the same effect can be obtained byuse of Ti or Ta or by stacking a plurality of materials selected fromTiN, Ti, and Ta.

[0054] As described and according to the invention, etching the topelectrode, the ferroelectric film and the bottom electrode with singlephotolithography process step does not cause short-circuit between thetop and bottom electrodes, thereby allowing the fabrication of thememory cell of a small cell area suitable for highly integrated memorydevices. Use of the memory cell according to the present invention canimplement not only a high integrated DRAM (Dynamic Random Access Memory)and a highly integrated ferroelectric nonvolatile memory, but also ahigh-performance LSI (Large Scale Integration) in which these memorycells and a logic LSI are integrated on one chip and afield-programmable logic LSI that allows modification of wiring by theferroelectric nonvolatile memory. It will be apparent that the effect ofthe present invention is by any means restricted to the memory cell ofthe above-mentioned embodiment; rather, the effect of the presentinvention extends to all semiconductor devices including LSIs forcommunications applications that use the ferroelectric capacitor.

[0055] While the preferred embodiment of the present invention has beendescribed using specific terms, such description is for illustrativepurposes only, and it is to be understood that changes and variationsmay be made without departing from the spirit or scope of the appendedclaims.

What is claimed is:
 1. A method of fabricating a semiconductor devicecomprising the steps of: forming a capacitor composed of a platinumbottom electrode, a dielectric film and a top electrode on a substrate;and etching said capacitor; wherein the etching is performed such thatan angle of each of main cross sectional sides of said dielectric filmrelative to a main surface of said substrate is not less than 45 degreesand not greater than 75 degrees.
 2. A method of fabricating asemiconductor device as claimed in claim 1 , wherein the etching isperformed such that an angle of each of main cross sectional sides ofsaid bottom electrode to the main surface of the substrate is not lessthan 45 degrees and not greater than 75 degrees.
 3. A method offabricating a semiconductor device as claimed in claim 2 , wherein theetching is performed such that an angle of each of main cross sectionalsides of said top electrode to the main surface of the substrate is notless than 45 degrees and not greater than 75 degrees.
 4. A method offabricating a semiconductor device as claimed in claim 1 , wherein theetching is performed such that an angle of each of main cross sectionalsides of said top electrode to the main surface of the substrate is notless than 45 degrees and not greater than 75 degrees.
 5. A method offabricating a semiconductor device as claimed in claim 1 , wherein saidangle of each of said main cross sectional sides of said dielectric filmrelative to said main surface of said substrate is not less than 60degrees and not greater than 75 degrees.
 6. A method of fabricating asemiconductor device as claimed in claim 5 , wherein the angle of eachof the main cross sectional sides of said bottom electrode to the mainsurface of the substrate is not less than 60 degrees and not greaterthan 75 degrees.
 7. A method of fabricating a semiconductor device asclaimed in claim 6 , wherein the angle of each of the main crosssectional sides of said top electrode to the main surface of thesubstrate is not less than 60 degrees and not greater than 75 degrees.8. A method of fabricating a semiconductor device as claimed in claim 5, wherein the angle of each of the main cross sectional sides of saidtop electrode to the main surface of the substrate is not less than 60degrees and not greater than 75 degrees.